Analog amplifiers normally have very low signal output impedances so that they can drive loads as constant voltage signal sources. Where more than one amplifier is employed to drive a common load, problems in load sharing develop. Such a situation can develop in a multiplex system where a plurality of signal sources are operated in a predetermined sequence to drive a common line. Where a plurality of signal amplifiers are coupled to a common line one way of control is to turn off the current flow in all but one amplifier. However, even when this is done some parasitic action can still be present. For example, in an output stage the input has stray shunt capacitance that will act to clamp the input potential. As the output varies the transistor bias varies. When the emitter-base diode is driven in the forward bias direction above one V.sub.BE the transistor will go into conduction. In the reverse bias state the emitter-base diode can be driven into zener breakdown at a relatively low level. Typically slightly over about six volts for a conventional integrated circuit (IC) transistor. Thus, the signal level must be low enough to avoid both forward conduction and zenering in all of the off amplifiers. Such a limitation is usually not acceptable in an operating system. Ordinarily signal channel switching is employed to selectively connect the desired driver amplifier to the common line. However, this requires highly conductive switching devices with one each for each amplifier channel. This kind of switching is commonly done with monopolar field effect transistors and their areas must be made quite large to obtain the required conductivity. It would be much more desirable to control the current flow inside the amplifiers themselves.